<scp>TGF</scp>‐β Enhances Doxorubicin Resistance and Anchorage‐Independent Growth in Cancer Cells by Inducing <scp><i>ALDH1A1</i></scp> Expression

The transforming growth factor-β (TGF-β)/Smad signaling pathway promotes malignant transformation through various mechanisms, and its effect on enhancing drug resistance can limit the efficacy of treatment. Here, we showed that pre-stimulation of human lung cancer A549 cells with TGF-β increases resistance to doxorubicin-induced growth inhibition in a Smad3- and Smad4-dependent manner. This effect was suppressed by the aldehyde dehydrogenase (ALDH) inhibitor oxyfedrine, suggesting that ALDH family members are involved in drug resistance. TGF-β upregulated the mRNA and protein expression of ALDH1A1. The TGF-β/Smad3 transcriptional enhancer region on ALDH1A1 was identified by Smad3 ChIP-seq analysis using an open database and by reporter assays. Knockdown of ALDH1A1 in A549 cells suppressed TGF-β-induced doxorubicin resistance, and lentivirus-mediated introduction of ALDH1A1 into A549 SMAD3-KO cells restored drug resistance. We also demonstrated that ALDH1A1 is required and sufficient for TGF-β/Smad3 signaling-induced anchorage-independent growth. The results suggest that the TGF-β/Smad3/4 axis promotes resistance to doxorubicin and anchorage-independent growth by inducing the transcription of ALDH1A1.